With the progress of electronics technologies in recent years, miniaturization, lighter weight, and lower power dissipation of electronic equipment have become possible along with sophistication of functions. As a result, a variety of cordless or portable consumer electronics equipment has been developed and commercialized and the market size is rapidly expanding. Typical examples include camcorders, lap top computers, and portable telephones. Further miniaturization and increasingly lighter weight as well as longer operating time of these equipment are always demanded. In association with this trend, there is a strong demand for continuing improvement in energy density and cycle life of small rechargeable batteries to be used in these equipment as a built-in power source. As built-in batteries, starting with lead-acid type and nickel-cadmium type batteries which were initially developed and commercialized, nickel-hydrogen (nickel-metal hydride system) storage batteries and lithium-ion secondary batteries which have higher capacity and higher energy density than these battery systems have subsequently been developed and commercialized. Among them, the lithium- ion secondary battery, which has a high energy density both per unit weight and per unit volume, is a battery system primarily using a complex oxide of lithium and a transition metal element as the positive electrode, a graphite-based carbon as the negative electrode, and a non-aqueous electrolyte such as organic electrolyte or polymer solid electrolyte as the electrolyte, and is recently enjoying a rapid growth in production. In this battery, during charge, lithium ions will desorb from the lithium-containing complex oxide of the positive electrode and transfer into the electrolyte, and at the same time, lithium ions of equal electrochemical equivalent will be fed from the electrolyte into the carbon of the negative electrode. Conversely, during discharge, lithium ions are fed to the positive electrode desorbing from the negative electrode. As this cycle is repeated, lithium-ion secondary battery is sometimes called a rocking-chair battery.
As the potential of the carbon negative electrode is close to the electrode potential of metallic lithium, a complex oxide of lithium which has a high electrode potential and a transition metal element is used as the positive electrode, for example, a complex oxide (LiCoO.sub.2 of lithium and cobalt, a complex oxide (LiNiO.sub.2) of lithium and nickel, and a complex oxide (LiM.sub.2 O.sub.4) of lithium and manganese. These complex oxides are often called as lithium cobaltate, lithium nickelate, and lithium manganate.
In the currently commercialized batteries, LiCOO.sub.2 which has a high potential and a long cycle life is most generally used as the positive active material. Under this situation, a use of LiNiO.sub.2 with which a higher capacity than tat of LiCoO.sub.2 is expected is now being actively studied. The reason for a higher capacity is because, as the electrode potential of LiNiO.sub.2 is lower than that of LiCoO.sub.2, it becomes possible to cause more quantity of lithium to desorb during charge before decomposition voltage of an aqueous electrolyte such as organic electrolyte is reached. As a result, the quantity of charged electricity is expected to increase, and hence the discharge capacity is also expected to increase.
Conversely, despite its large initial discharge capacity, LiNiO.sub.2 suffers a problem of cycle deterioration by gradual decrease in the discharge capacity as charge and discharge are repeated.
As a result of disassembling a battery cell of which the discharge capacity has decreased due to repeated charge-discharge cycles and X-ray diffraction analysis of the positive active material by the inventors of this invention, a change of crystal structure was observed after charge and discharge cycles, which was confirmed to be the cause for deterioration.
Similar phenomenon has already been published by W. Li, J. N. Reimers and J. R. Dahn in Solid State Ionics, 67, 123 (1993). It is reported in this paper that, with the repetition of charge and discharge, lattice constant of LiNiO.sub.2 changes while itself changes from a hexagonal to a mono-clinic system crystal, and further from a second hexagonal to a third hexagonal system crystal as lithium is desorbed. This type of change in crystal phase lacks reversibility and as charge-discharge reactions are repeated, the sites where insertion and desorption of lithium are possible are gradually lost. This phenomenon is considered to be the cause of decrease in the discharge capacity.
In contrast, with LiCoO.sub.2, such a change in the crystal phase as described above on LiNiO.sub.2 will not occur in the region of normal voltage (a voltage at which an organic electrolyte oxidizes and decomposes), suggesting that a decrease in the discharge capacity due to charge-discharge cycles is not likely to take place.
For the purpose of addressing the problem of decrease in discharge capacity of LiNiO.sub.2 due to charge-discharge cycles, many proposals have heretofore been made to substitute a part of the element Ni with transition metal elements, primarily Co.
As an example, in Japanese Laid-open Patent No. Sho 62-256,371, a method of synthesizing a Li-containing complex oxide of Co and Ni by firing at 900.degree. C. a mixture of lithium carbonate (LiCO.sub.3, cobalt carbonate (CoCO.sub.3), and nickel carbonate (NiCO.sub.3).
Methods of synthesizing complex oxides are disclosed in Japanese Laid-open Patent No. Sho 62-299,056 in which a mixture of carbonates, hydroxides, and oxides of Li, Co, and Ni is used as the raw material, and in U.S. Pat. No. 4,980,080 in which a mixture of lithium hydroxide (LiOH), Ni oxides and Co oxides is used as the raw material, and both heated at 600.degree. C. to 800.degree. C.
In addition, in U.S. Pat. No. 5,264,201 and other patents, methods of synthesizing Li-containing complex oxides are disclosed in which lithium hydroxide (LiOH) is added to and mixed with a uniform mixture of oxides or hydroxides of Ni and oxides or hydroxides of Fe, Co, Cr, Ti, Mn, or V, followed by heat treatment at a temperature not lower than 600.degree. C.
Furthermore, in Japanese Laid-open Patent No. Hei 1-294,364 and other patents, methods of synthesizing Li-containing complex oxides are disclosed in which carbonates, more precisely basic carbonates, of Ni and Co are co-precipitated from an aqueous solution containing Ni ions and Co ions, and a mixture of the co-precipitated carbonates and Li.sub.2 CO.sub.3 is fired.
There have also been proposed a method in which a Ni-containing oxide and a Co-containing oxide are mixed and fired after being further mixed with carbonates or oxides of Li, and a method in which a Li-containing complex oxide is synthesized by using oxides containing both Ni and Co such as NiCo.sub.2.
These inventions represent efforts to relax a change in crystal phase by substituting a part of Ni with Co or other transition metal elements. The reason why there are many proposals to substitute part of Ni with Co from among different transition metals is because substitution is easy as the ion radius of Co is approximately equal to that of Ni and that the bond strength of Co with oxygen is stronger than that of Ni, which suggest that the crystal structure may become more stable and that the decrease in discharge capacity due to charge-discharge cycles may be improved.
However, not all of the three battery characteristics, namely, discharge capacity, cycle life characteristic, and reliability as a battery, could be satisfied by the Li-containing complex oxides represented by a chemical formula Li.sub.x Ni.sub.y Co.sub.z O.sub.2 (0.90.ltoreq..times..ltoreq.1.10, 0,7.ltoreq.y.ltoreq.0.95, y+z=1) as obtained by the methods of synthesis heretofore been disclosed or proposed. For example, even though a positive electrode produced by using a Li-containing complex oxide synthesized from a mixture of each respective carbonates, hydroxides, or oxides of Li, Co, and Ni did have a certain large initial discharge capacity, the discharge capacity decreased with charge-discharge cycles though not as markedly as with LiNiO.sub.2 of which no Ni had been substituted with Co and it was not satisfactory as a positive active material.
A close study of the causes of such performance deterioration by the inventors of the present invention has revealed that, in the conventional methods of production, as the quantity (value of z) of Co substituting Ni increases (z.gtoreq.0.1), in reality, Ni and Co are not uniformly dispersed in the obtained complex oxide, partially leaving LiNiO.sub.2 and LiCoO.sub.2 as a mixture. It has been found that although a positive electrode made from these active materials shows a somewhat large discharge capacity, the part not properly substituted with Co had caused a change in the crystal phase with repeated charge and discharge, thus damaging the crystal structure and loosing reversibility, resulting in the decrease of discharge capacity.
To address the non-uniform dispersion of Ni and Co in such active materials, a method of producing a positive active material using as the raw material carbonates co-precipitated from an aqueous solution of a mixture of Ni ions and Co ions is proposed in Japanese Laid-pen Patent No. Hei 1-294,364. With this method, it is true that a carbonate in which Ni and Co are uniformly dispersed can be obtained. However, when this co-precipitated carbonate is mixed with Li.sub.2 CO.sub.3 and fired, the co-precipitated carbonate is first decomposed generating a large volume of carbon dioxide (CO.sub.2) causing an increase in the CO.sub.2 partial pressure of the firing atmosphere. It further causes a decrease in the partial pressure of oxygen (O.sub.2) which is necessary for producing a complex oxide, and blocks the progress of the synthesizing reaction. Consequently, unless some forcible means of increasing O.sub.2 partial pressure is adopted, it has been difficult to obtain a perfect complex oxide. As a result, when using a Li-containing complex oxide synthesized by conventional methods of production, although the deterioration of discharge capacity due to charge-discharge cycles may be low, the initial discharge capacity is not necessarily high and is considered to be insufficient. Furthermore, because of the presence of CO.sub.2 during synthesis, Li.sub.2 CO.sub.3 remains mixed as an excess lithium salt after synthesis has been completed. When a cell which uses an active material containing this Li.sub.2 CO.sub.3 is stored at a high temperature of 80.degree. C. in a charged state, Li2CO.sub.3 in the positive electrode decomposes and releases CO.sub.2, thus causing an increase in the internal cell pressure. Therefore, this production method has not been put into practice.
In addition to these methods, there is a method to use NiCoO as a synthesizing material and lithium oxide (Li.sub.2 O) as a source of supply of Li. However, as the melting point of Li.sub.2 O is not lower than 1,700.degree. C., the reactivity is low and a perfect Li-containing complex oxide could not be synthesized. Consequently, the initial discharge capacity of a cell using a positive electrode made of this complex oxide was not satisfactory.
As a means for uniformly dispersing Ni and Co, apart from the use of co-precipitated carbonates of Ni and Co or NiCoO.sub.2, there exists a method of co-precipitating them as hydroxides, or a method of production by mixing a Ni hydroxide in which Co is dissolved as a solid solution and a lithium salt and then firing. In this production method, too, the hydroxide as the raw material decomposes first and a large volume of 120 is generated. Since the H.sub.2 O partial pressure increases, it is difficult to maintain the O.sub.2 partial pressure of the firing atmosphere at a level appropriate to synthesize a perfect Li-containing complex oxide as in the case of use of a co-precipitated carbonate as the raw material, and hence the progress of synthesizing reaction is blocked.
Consequently, a co-precipitated hydroxide of Ni and Co is first thermally decomposed at 190.degree. C. to 250.degree. C. to prepare (Ni.sub.y Co.sub.1-y).sub.3 O.sub.4 or Ni.sub.y Co.sub.1-y O. There is a published report of mixing LiOH to these oxides and firing at 450.degree. C. (Ref. J. Solid State Chemistry, 113, 182-192(1994)). In this method, presumably because the firing temperature during synthesis was relatively low, nonreacted lithium compounds (Li2CO3 and LiOH) were observed by X-ray diffraction analysis when the quantity of substitution of Ni with Co was not greater than 60 mol %. Consequently, it was not possible to synthesize a perfect Li-containing complex oxide even with this method.
The present invention aims at addressing various problems encountered in using Li-containing complex oxides synthesized by the heretofore proposed or reported methods of production as positive active materials. That is, as a result of studying conventional methods of production in detail, the present invention provides a new method of production of positive active material for a high-reliability non-aqueous electrolyte secondary battery of which the discharge capacity is high and the decrease of the discharge capacity due to charge-discharge cycles is controlled by using in the positive electrode a Li-containing complex oxide obtained by using nickel oxides in which a specific element has been dissolved as a solid solution as a synthesizing material, and firing it using LiOH or a mixture of LiOH and its hydrates as the source of supply of Li.